This invention relates to a slip ring assembly used to transmit electrical signals from a rotating shaft to a stationary system, and more particularly, to a internal wiring arrangement for a slip ring assembly which minimizes electrical noise due to rotational effects at high speeds.
Typically, high speed slip rings are used to connect sensors on rotating components to stationary data acquisition equipment. The sensors are usually temperature sensors or strain gages. Each sensor requires two or more independent contacts on the slip ring to make a complete circuit. According to Ohm's Law, linear, homogeneous, isotropic mediums exhibit the characteristic that the voltage equals the current multiplied by the resistance for any closed loop (V=IR). Sensors make use of this law by holding either the voltage or the current constant and measuring the change in the other. The change in current or voltage which the circuit measures is the output of the sensor. A common problem, however, is that the surroundings of the slip ring, or even the slip ring itself, may contain magnetic fields. While the slip ring shaft is rotating, the current goes in on one contact and returns on another to complete a circuit. The result is a current loop which rotates through magnetic flux lines. This phenomena induces an alternating current in the circuit. In the case of a slip ring, this alternating current appears as undesirable background noise which decreases the clarity of the sensor signal. On the output, the alternating current is a sine wave with the period equal to the period of rotation of the slip ring shaft.
Another factor of importance is the internal resistance of the circuit. All conductors have a resistivity, .rho., which is characteristic of the material. The resistance of any conductor may then be calculated using the relationship of R=.rho.L/A, where L is the length of the conductor and A is the cross-sectional area. The internal resistance of a circuit may then be calculated using the path length that the electrical signal must travel. In the case of a slip ring circuit, the current enters the slip ring at the brush-ring interface, continues through the sensor and then leaves the slip ring at another brush-ring interface. Continuity from the ring to the sensor, however, is made at only one point on the ring. The result of this is that as the ring rotates and the brush-ring contact point remains stationary, the electrical signal path length changes, and as the path length changes, the internal resistance of the circuit changes. This varying resistance causes an alternating current on the output signal and its shape is determined by the number of brush-ring contact points for each channel, and the angular orientation of the ring-wire contact points for each ring. This once per revolution signal interferes with the output generated by the sensor.